WordPress at a glance
action is not described

manage_posts_extra_tablenav action-hook . WP 4.4.0

Fires immediately following the closing "actions" div in the tablenav for the posts list table.

Usage

add_action( 'manage_posts_extra_tablenav', 'action_function_name_4228' );
function action_function_name_4228( $which ){
	// action...
}
$which(string)
The location of the extra table nav markup: 'top' or 'bottom'.

Where the hook is called

WP_Posts_List_Table::extra_tablenav()
wp-admin/includes/class-wp-posts-list-table.php 502
do_action( 'manage_posts_extra_tablenav', $which );

Where the hook is used (in WP core)

Does not used.